The present invention relates to a system and method for protecting against the overheating of solenoids due to electrical faults.
Intermittently rated solenoids (i.e., of the type designed to be pulse operated) can overheat when they are inadvertently operated continuously. This can occur, for example, when a faulty control signal or an electrical short causes the solenoid to be continuously energized. Prior systems have attempted to address this issue by incorporating over-temperature protection within the windings of each solenoid in the circuit. This is not always possible due to space constraints—particularly where solenoids are tightly packed as the case in certain printers, such as drop on demand ink jet printers. This also adds significant cost and potential unreliability to a printer. Furthermore, this technique still risks high temperatures and potential damage to the solenoid before the over-temperature detection can trip out due to inherent thermal lags in this type of protection.
FIG. 1A is schematic of a prior art solenoid control and protection system. The system includes a power supply 10, a controller 14, a plurality of solenoids 18a-18n and a plurality of solenoid control switches 22a-22n. The power source 10 produces a predetermined voltage across its terminals 24, 26. The solenoids 18a-18n are connected in parallel across the terminals 24, 26 of the power source 10 for receiving power therefrom. In this regard, each solenoid 22a-22n has a respective coil having a first terminal 30 coupled to the supply terminal 24 of the power source 10 and a second terminal 32 coupled to the return terminal 26 of the power source 10 through a respective control switch 22a-22n. The controller 14 energizes/deenergizes the solenoids 18a-18n by connecting/disconnecting the solenoids to/from the power source 10 via the switches 22a-22n. 
The system also includes a plurality of current sensors 34a-34n, each of which is connected downstream of a respective control switch 22a-22n for detecting a current flow through a respective solenoid 18a-18n and producing a signal responsive thereto. Each current sensor 34a-34n includes a resistor 36 and a differential amplifier 38. The resistor 36 is connected between a respective solenoid control switch 22a-22n and the return terminal 26 of the power source 10. The inputs of the amplifier 38 are connected across the resistor 36. The output of the amplifier produces a voltage (signal) indicative of the voltage across the resistor 36, and, hence, the level of current flowing through the resistor 36.
The controller 14 is coupled to the current sensors 34a-34n for receiving the outputs of the amplifiers 38. The controller 14 processes these signals to detect certain faults in the system. For example, an open circuit may occur between a solenoid and its respective control switch. The controller 14 can detect such an open circuit, e.g., by detecting the lack of current flow when the switch is closed. While the system of FIG. 1 can detect some faults, it is still possible for certain faults to go undetected and result in undesirable solenoid overheating. For example, if the second terminal 32 of a solenoid shorts to ground (e.g., to a printer chassis), the corresponding switch and sensor will be bypassed and the solenoid will be permanently energized with complete loss of control. This is shown in FIG. 1B, where the solenoid 18n is shorted to ground.